1. Technical Field of the Invention
The present invention relates to an optical beam scanning apparatus and a method of manufacturing an optical beam scanning apparatus and to an image forming apparatus provided with this optical scanning apparatus and a method of manufacturing an image forming apparatus. In particular, the invention relates to an optical beam scanning apparatus which in an overillumination scanning optical system in which a width of a luminous flux made incident on a polygon mirror is wider than a width of one reflecting surface forming the polygon mirror, is capable of scanning the luminous flux on a photoconductive drum and a method of manufacturing an optical beam scanning apparatus and to an image forming apparatus provided with this optical beam scanning apparatus and a method of manufacturing an image forming apparatus.
2. Related Art
In recent years, in image forming apparatus of an electrophotographic mode, for example, laser printers, digital copiers and laser facsimiles, an optical beam scanning apparatus for irradiating laser light (optical beam) on a surface of a photoconductive drum and scanning the laser light to form an electrostatic latent image on the photoconductive drum is provided.
Recently, in order to devise to realize high-speed scanning on a surface of a photoconductive drum, for example, a method in which plural light sources (laser diodes) are provided in one laser unit, thereby increasing the number of laser light per one scanning (multibeam method) is proposed. In this multibeam method, plural beams for every color component emitted from each of light sources (for example, yellow, magenta, cyan, and black) are processed in a pre-deflection optical system and converted into one beam, which is then made incident on a polygon mirror. The beam deflected by the polygon mirror is mediated through an fθ lens configuring a post-deflection optical system and subsequently separated into a beam for every color component and irradiated on a photoconductive drum of every color component.
Here, the rotation axis direction of the polygon mirror as a deflector is defined as “sub-scanning direction”, and a direction vertical to each of the optical axis direction of the optical system and the rotation axis direction of the deflector (polygonal mirror) is defined as “main scanning direction”. Incidentally, the sub-scanning direction in the optical system is corresponding to a conveyance direction of a transfer material in an image forming apparatus, and the main scanning direction in the optical system is corresponding to a direction vertical to the conveyance direction within a surface of the transfer material in the image forming apparatus. Also, an image surface shows the surface of the photoconductive drum, and an imaging surface shows a surface on which a luminous flux (laser light) actually forms an image.
In general, a relation expressed by [Expression 1] is present among an image processing rate (paper conveyance rate), an image resolution, a motor rotation rate and a number of polygon mirror surfaces.
                              P          *          R                =                              25.4            *            Vr            *            N                    60                                    [                  Expression          ⁢                                          ⁢          1                ]            
In the foregoing expression, P (mm/s) represents a processing rate (paper conveyance rate); and R (dpi) represents an image resolution (number of dots per inch). Also, Vr (rpm) represents a number of revolutions of a polygon motor; and N represents a number of polygon mirror surfaces.
As expressed by the foregoing [Expression 1], the printing speed and resolution in the image forming apparatus are proportional to the number of revolutions of a polygon motor (Vr) and the number of polygon mirror surfaces (N). Accordingly, in order to realize high resolution as well as high speed in the image forming apparatus, it is necessary to increase the number of polygon mirror surfaces (N) or to raise the number of revolutions of the polygon motor (Vr).
However, in a conventional general underillumination scanning optical system, a width of a luminous flux (laser light) made incident on a polygon mirror in a main scanning direction is made smaller than a width of one reflecting surface forming the polygon mirror in the main scanning direction (reflection width) thereby reflecting the whole of the luminous flux (laser light) made incident on the polygon mirror.
However, since not only a beam diameter on the image surface is proportional to an F number, but also the F number Fn is expressed by Fn=f/D wherein f represents a focal distance of the imaging optical system, and D represents a beam diameter of the main scanning direction on the polygon mirror surface, when it is intended to make the beam diameter on the image surface small for the purpose of devising to realize high image quality, the beam diameter of the main scanning direction on the polygon mirror surface must be made large.
In other words, in order to obtain high image quality at a certain level or more, there is a restriction that the beam diameter of the main scanning direction on the polygon mirror surface must be regulated to a fixed size or more.
Nevertheless, in order to realize high resolution as well as high speed, when it is intended to increase the number of polygon mirror surfaces (N), the polygon mirror itself must be increased in size. As a result, when it is intended to rotate a large-sized polygon mirror at a high speed, a load to a motor for driving the polygon mirror becomes large, and the motor cost increases. In addition, at the same time, the noise or vibration of the motor or the generation of a heat becomes large, and a countermeasure thereto becomes necessary separately.
Then, an image forming apparatus using an over-illumination scanning optical system is proposed in place of the underillumination scanning optical system. In the overillumination scanning optical system, a width of a luminous flux made incident on a polygon mirror is made wider than a width of one reflecting surface forming the polygon mirror.
According to this, it is possible to reflect the luminous flux by using the entire surface of the reflecting surface forming the polygon mirror (or plural reflecting surfaces); and even in the case where it is intended to ensure the beam diameter on the polygon mirror surface while increasing the number of reflecting surfaces of polygon mirror (N) for the purpose of devising to realize high resolution as well as high speed, it is possible to make the diameter of the polygon mirror itself small. Accordingly, a load to a motor for driving the polygon mirror can be reduced, and the motor cost can be reduced. Also, since not only the diameter of the polygon mirror itself can be made small, but also the number of reflecting surfaces can be increased, it is possible to make the shape of the polygon mirror close to a circle, and it is possible to make the air resistance at the time of driving the polygon mirror low. As a result, even when the polygon mirror is rotated in a high speed, it is possible to reduce the noise or vibration and the generation of a heat.
Furthermore, following the reduction in the noise or vibration and the generation of heat, the whole or a part of countermeasures element for reducing the noise or vibration, such as glasses, become unnecessary, and the costs in manufacturing an image forming apparatus can be lowered. Also, a high duty cycle becomes possible.
The foregoing overillumination scanning optical system is described in, for example, Leo Beiser, Laser Scanning Notebook, SPIE OPTICAL ENGINEERING PRESS.
In the case where an imaging lens is manufactured by using a resin as a material of an imaging lens included in a post-deflection optical system and molding the resin into a prescribed shape, gate cutting becomes necessary. But, a residual stain or deformation is generated in the vicinity of the gate cut part of the manufactured imaging lens due to a heat at the time of gate cutting.
For example, in the case where the lens of a post-deflection optical system is a molded lens which is manufactured by molding a resin which has been allowed to flow into a molding die through a side gate opening, a residual strain or deformation or the like is generated in an end of the imaging lens in the side corresponding to the side of the gate opening or the like due to a heat at the time of gate cutting.
When a luminous flux (laser light) passes through a gate cut part of the lens in which a residual strain or deformation or the like has been generated due to a heat at the time of such gate cutting, the beam diameter on the image surface becomes large as compared with the usual.
In particular, in the case where the lens of a post-deflection optical system is configured of a single lens, since the lens requires a larger power, it is necessary that the lens is a thick-wall lens. For that reason, the cross-sectional area of the lens to be molded in the vicinity of the side gate opening for making a resin flow therein (gate cut part of the lens) becomes large so that when it is intended to gate cut this portion, a residual strain or deformation is more likely generated in the lens due to a heat at the time of gate cutting. As a result, when a luminous flux (laser light) passes through the gate cut part of the lens, the beam diameter on the image surface becomes larger as compared with the usual.
Here, in the conventional underillumination scanning optical system, a width of a laser beam L deflected by the polygon mirror corresponding to the main scanning direction was constant irrespective of the scanning position (angle). However, in the overillumination scanning optical system, the width of the laser beam L corresponding to the main scanning direction fluctuates depending upon the scanning position (angle).
Concretely, in the case where the optical axis of the laser beam L made incident on the polygon mirror and the optical axis of the post-deflection optical system form an angle, the width of the luminous flux corresponding to the main scanning direction fluctuates depending upon the scanning position (angle).
For that reason, the F number varies depending upon the scanning position (angle); and when the laser beam is made incident on the polygon mirror, as it goes from a light incidence side to an opposite side to the light incidence side, the beam diameter of the main scanning direction on the image surface becomes large, thereby generating scattering in beam diameter of the main scanning direction on the image surface. In other words, the beam diameter of the main scanning direction on the image surface is bilaterally asymmetric against the optical axis of the optical system in a scanning region on the photoconductive drum; and when the laser beam L is made incident on the polygon mirror, as it goes from a light incidence side to an opposite side to the light incidence side, the optical characteristics on the image surface become worse.
For that reason, in the case of manufacturing an imaging lens by molding a resin, when the side gate is used, if the gate cut part of the imaging lens is arranged in the opposite side to the light incidence side, in the opposite side to the light incidence side, in addition to the worseness of optical characteristics on the image surface, the beam diameter on the image surface becomes very large due to influences of a residual strain or deformation generated at the time of gate cutting.
As a result, scattering in the beam diameter in a scanning region of the photoconductive drum as an image surface becomes large.